500 



COLLEGE ZOOLOGY 



(Fig. 70). These muscle fibers, like those in 

 higher animals, are mesodermal in origin. 

 The body wall of an annelid is very mus- 

 cular, containing an outer circular layer and 

 an inner longitudinal layer (Fig. 92). Spe- 

 cial muscles are present for moving the 

 setae, and in the polychaetes, for moving 

 the parapodia as well. The setae are chiti- 

 nous skeletal structures that aid in locomo- 

 tion. Leeches and certain other parasitic 

 types, such as trematodes, are provided with 

 muscular sucking disks that are used for 

 attachment and for purposes of locomotion. 

 In many mollusks, locomotion is due to 

 the activity of muscles in the foot. In the 

 bivalves the shell may be closed by trans- 

 verse adductor muscles. The tube feet of 

 echinoderms are highly specialized loco- 

 motor organs, which are extended by a fluid 

 driven into them by the contraction of cir- 

 cular muscles in the walls of the ampullae, 

 inside the shell, and contracted by longi- 

 tudinal muscles in their own walls. 



Movements with 

 an exoskeleton 



In arthropods with an exoskeleton, circu- 

 lar and longitudinal muscles such as occur 

 in annelids are replaced by special muscles 

 which extend from one segment of the 

 body to another, or between the joints of 

 the appendages, and are fastened to the in- 

 side of the shell. In the cravfish, sudden con- 

 tractions of the powerful flexor abdominal 

 muscles bend the abdomen forward and 

 drive the body backward. Crayfishes are able 

 to walk in any direction by means of muscles 

 within the legs that act much like those in 

 the legs of insects. In the grasshopper and 

 in most other insects, wings as well as legs 

 serve as locomotor organs. Wings may be 

 raised by contraction of tergosternal muscles 

 and lowered by contraction of dorsolongi- 

 tudinal muscles, as shown in Fig. 135. The 

 rate of wing beat may be as low as 9 strokes 

 per second in butterflies and as high as 330 

 strokes per second in the house fly. 



INTERNAL SKELETONS 

 OF VERTEBRATES 



Skeletons of aquatic 

 vertebrates 



The vertebrate skeleton gives support and 

 protection to the body and furnishes a firm 

 surface for muscle attachment. Aquatic 

 vertebrates differ from terrestrial vertebrates 

 in the character of both the axial and ap- 

 pendicular sections of the skeleton. Since 

 they are held up by water, aquatic verte- 

 brates neither need nor possess the sturdy 

 type of skeleton required in land animals 

 for holding the body above the ground. The 

 appendages of fish are paired pectoral and 

 pelvic fins, which contain weak skeletal 

 structures (Fig. 254); they serve primarily 

 for balancing and steering rather than for 

 locomotion. Other types of vertebrates such 

 as frogs, salamanders, turtles, alligators, pen- 

 guins, herons, whales, and seals have skele- 

 tons variously adapted to an aquatic exist- 

 ence. 



Human skeleton 



In general the skeleton of man resembles 

 that of the frog and has similar functions. 

 Some of the principal bones are labeled in 

 Fig. 369; the parts of the skull are shown 

 in Fig. 370, and of a vertebra in Fig. 371. 

 The cranium consists of 8 bones: occipital, 

 parietal (2), frontal, temporal (2), eth- 

 moid, and sphenoid. The 14 bones of the 

 face are the nasal (2), vomer, inferior nasal 

 conchae (inferior turbinate) (2), lacrimal 

 (2), zygomatic (2), palatine (2), maxilla 

 (2), and mandible. In each ear are 3 bones, 

 the malleus, incus, and stapes; and in the 

 neck a single hyoid bone. The vertebral 

 column averages about 28 inches long; it 

 consists of approximately 33 bones, but in 

 adults 5 are fused to form the sacrum, and 4 

 to form the coccyx.* The sacrum and coc- 



* Tlie number of coccygeal bones varies from 3 

 to 5. Tliey represent bony remnants of the tail of 

 lower animals. 



